This invention relates generally to transit-time sonic flowmeters used to measure the flow of liquids in full pipes and more particularly concerns an autoranging flowmeter that automatically determines pipe inside diameter and uses the diameter value in calculations of flowrate.
A typical transit-time sonic flowmeter is described in U.S. Pat. No. 3,720,105, Acoustic Flowmeter, issued to Uldis Cirulis and is further described in an article by Ellis M. Zacharias and Donald W. Franz entitled "Sound Velocimeters Monitor Process Streams" published in the Jan. 22, 1973 issue of Chemical Engineering (pages 6-8 and FIG. 5 of article reprint).
In a typical sonic flowmeter two transducers are positioned so that they may communicate by alternately sending and receiving acoustic signals through the liquid that flows through the pipe. The transducers are so positioned that the line between them usually intersects the pipe axis at an angle of 45 degrees, but the angle may vary from 30 to 60 degrees, depending upon the circumstances. A small angle is desirable because it will result in better flow sensitivity. The transit time of an acoustic signal between transducers will depend on transducer spacing, the velocity of sound in the liquid and the rate of flow of the liquid. Thus, if the spacing and sound velocity are known, the rate of flow can be determined.
There are several problems inherent in the known acoustic flowmeter systems. For example, the transducers are generally recessed back from the inside wall of the pipe. Therefore, the time T it takes an acoustic signal to traverse the path from one transducer to the other includes the times through the non-flowing column of liquid in each recess as well as the time through the liquid flowing in the pipe T.sub.p. Inaccuracy in compensating for these time components results in inaccuracy in the metering system. Another problem with known systems is that a different pair of transducers is generally dedicated to monitor each flow station, resulting in higher system costs. In addition, for very narrow pipes, problems ensue because the separation between transducer faces is minimal and the transit time is brief. When the transit time is brief, a transducer may assume the role of a receiver before it has become quiet following excitation as a transmitter and the self noise that results may create an error in the determination of arrival time of an acoustical signal from the opposing transducer. Finally, most systems require a relatively high voltage source because a low voltage excitation signal results in a proportionately smaller acoustic signal, which in turn decreases the signal-to-noise ratio at the receiving transducer, making it difficult to use the flowmeter on larger diameter pipes.
It is, therefore, an object of this invention to provide a sonic flowmeter which determines pipe size by measuring the transit time and subtracting out those components of transit time that are not pipe diameter dependent. It is a further object of this invention to provide a sonic flowmeter which contains acoustic windows that permit the transducers to be withdrawn and used elsewhere. It is also an object of this invention to provide a sonic flowmeter in which a single transmission is followed by a waiting period prior to an opposite transmission to reduce noise problems in the system, or, as an alternative, to provide a sonic flowmeter in which each transducer may transmit a succession of signals before reverting to the role of a receiver. And it is an object of this invention to provide a sonic flowmeter which generates a comparatively high (80 volts) transducer excitation from a low voltage (9 volt battery) source.